After install, louder fans and strange hum

I recently replaced the Hard Drive in my iMac desktop and have noticed increased noise that comes and goes from time to time. It swells in volume and then slowly decreases and goes away completely, but it gets loud enough for me to worry. I'm hopping it is something that simply needs to settle. Any ideas? Maybe a fan is not balanced or needs oil? I did attempt to vacuum the two fans that were visibly very dusty, but I didn't get too rough with them. When trying to pinpoint the noise, it seems to come from the left hand side of the screen when facing the display side.

Quick update: The computer is running great again and noise seems to have gone away. I hope it does not mean a dead or frozen fan. I've notice that after leaving the computer idle for a few hours, the majority of the heat coming out of the rear top vent is from the left while the right is considerably cooler with hardly any air escaping (DVD drive side). This could be normal though as I have never really check this computer for proper ventilation prior to this. Any thoughts would still be appreciated, thanks!

What HD did you use to replace your old one? Was the SATA I/O speed the same? Your original drive was a SATA II (3.0 Gb/s) you need to replace it with the SAME I/O speed drive or one that can be adjusted to SATA II, or the slower SATA I (1.5 Gb/s). A SATA III (6.0 Gb/s) will appear to work but will cause the system to over heat as the CPU & HD will be fighting with high retries (reads & writes).

Since you noted less of a problem after letting your system sit for a while is a symptom of a mismatched SATA I/O HD problem. You see the indexer process needs to re-index all of your files you just installed or copied over. Once the index has managed to complete the HD I/O will be much less.

As to cleaning you need to be careful with vacuums as they tend to be mini Van de Graaff generators. Which can be very deadly to computer systems. You should get a can of Can'ed air to blow the build up away after using a soft paint brush to carefully scrub the dust off. Don't spin up the blades to high or long as you can damage the bearings. As lubing the fan bearings you really can't and any oil that is exposed will collect dust and will foulup the fan.

Yes, I did follow the iFixit guide as well as the OWC video tutorial side by side and really took my time with it. I replaced the old 750GB with a 1TB Seagate Hybrid (SSD/HDD) SATA 6GB. The noise is now complete gone and the computer is running wonderfully. However, the thing that still bothers me is that the sound was not of simply fans revving to their max while re-indexing was running, but more of a bearing misalignment type of sound. I was very careful with the vacuum around all components and did not let the fans rev from the suction. In the future, I will definitely use a paint brush (brilliant idea)!

I think I will be best served to monitor the temps as best I can via software and see if anything gets too hot. I know prior to repair, this computer tended to get very hot and was very unwilling to rev the fans to dissipate the air, so this one is going to be tough.